This invention relates to an engine incorporating means to combat air pollution from its exhaust gas and to a method of converting pollutants in the exhaust gas of an engine in order to combat air pollution. The engine may be a stationary engine but is especially a vehicle engine. The engine may be powered by petrol (gasoline), diesel, natural gas or other hydrocarbon or oxygenate fuel. The invention will be described with particular reference to petrol fuelled engines, but is not to be considered to be limited thereto.
The main pollutants in the exhaust gas of an engine such as a petrol engine are carbon monoxide (CO), hydrocarbons and nitrogen oxides. The amount of these pollutants which is emitted in the exhaust gas into the air is generally reduced by means of catalysts in the exhaust apparatus of the engine. CO is converted to CO2 by a CO oxidation catalyst. Hydrocarbon is converted to CO2 and water by a hydrocarbon oxidation catalyst. Nitrogen oxides are converted to nitrogen by a nitrogen oxides reduction catalyst. A so-called three-way catalyst converts CO, hydrocarbon and nitrogen oxides in this way. Three-way catalysts are composed of a mixture of catalytically active materials, at least one being active for the conversion of CO and hydrocarbons and at least one for the conversion of nitrogen oxides. Three-way catalysts are generally based on rhodium admixed with platinum and/or palladium.
As regulations governing the amount of pollutants which may be emitted from engines such as petrol engines have become stricter, attention has been focussed on the start-up phase from ambient temperature. For present purposes, ambient temperature may be defined as 25xc2x0 C. Emissions of hydrocarbons are highest in this phase because the hydrocarbon oxidation catalyst has not warmed up to its operating temperature. The xe2x80x9clight-offxe2x80x9d temperature is the temperature at which 50% of the pollutant is converted. On starting an engine at ambient temperature, the time taken for the hydrocarbon oxidation catalyst to warm up to its light-off temperature is significant, and in that time a significant amount of hydrocarbon is emitted into the air. Various approaches have been proposed for reducing this cold-start emission of hydrocarbon, including:
(a) trapping hydrocarbon at low temperatures and releasing it at high temperatures;
(b) electrically heating the catalyst;
(c) close-coupling the catalyst, whereby the catalyst is positioned very close to the engine in order to benefit well from engine heat; and
(d) employing a CO oxidation catalyst which lights off below ambient temperature in combination with an engine and exhaust apparatus adapted so that the exhaust gas contacting the catalyst contains sufficient oxygen and sufficient CO and/or hydrogen that the exothermic reaction of the oxygen with the CO and/or hydrogen generates enough heat to raise the catalyst to at least the light-off temperature of the hydrocarbon oxidation catalyst so that the hydrocarbon oxidation catalyst is at a temperature of at least its light-off temperature.
Emission regulations, however, are becoming ever stricter. In the United States, the hydrocarbon limit is tightening from the 0.04 g per mile (1.6 km) (as measured over the Federal Test Procedure cycle) ULEV limit to the newly proposed SULEV limit of 0.008 g per mile (1.6 km) and EZEV limit of 0.004 g per mile (1.6 km). The present invention has for an object the achievement of very stringent hydrocarbon emission standards.
The invention provides an engine producing exhaust gas containing CO, H2 and hydrocarbon, the engine having:
(a) exhaust apparatus through which the exhaust gas flows;
(b) means to supply air to the exhaust apparatus; and
(c) engine management means;
the exhaust apparatus containing:
(d) an electrical heater;
(e) a CO and H2 oxidation catalyst for oxidising CO and H2 in the exhaust gas, the CO and H2 oxidation catalyst being positioned on or downstream of the electrical heater; and
(f) a hydrocarbon oxidation catalyst for oxidising hydrocarbon in the exhaust gas, the hydrocarbon oxidation catalyst being also the CO and H2 oxidation catalyst or being positioned downstream thereof;
the engine being adapted so that on starting it at ambient temperature, the engine management means is effective to produce heat electrically by the electrical heater, and so that on or after the start of engine and extending into a time at least 5 seconds after the start of the engine, the engine management means is effective to decrease the air/fuel ratio to the engine so as to increase the amount of CO and H2 supplied to the CO and H2 oxidation catalyst and to supply sufficient air to the CO and H2 oxidation catalyst by the means to supply air so as to increase the amount of CO and H2 oxidised by the CO and H2 oxidation catalyst and hence increase the heat produced chemically by the CO and H2 oxidation catalyst, whereby the heat produced electrically by the electrical heater and the increase in heat produced chemically by the CO and H2 oxidation catalyst heat the CO and H2 oxidation catalyst to speed up its reaching the hydrocarbon light-off temperature of the hydrocarbon oxidation catalyst and hence speed up the hydrocarbon oxidation catalyst reaching its light-off temperature.
The invention provides also a method of converting CO, H2 and hydrocarbon in the exhaust gas of an engine to CO2 and water in order to combat air pollution, by contacting the gas with a CO and H2 oxidation catalyst and simultaneously or subsequently with a hydrocarbon oxidation catalyst, which method is conducted so that on starting the engine at ambient temperature an electrical heater produces heat electrically and on or after the start of the engine and extending into a time at least 5 seconds after the start of the engine, the air/fuel ratio of the engine is decreased and air is supplied so as to increase the amount of CO and H2 supplied to the CO and H2 oxidation catalyst and to supply sufficient air to the CO and H2 oxidation catalyst so as to increase the amount of CO and H2 oxidised by the CO and H2 oxidation catalyst and hence increase the heat produced chemically by the CO and H2 oxidation catalyst, whereby the heat produced electrically by the electrical heater and the increase in heat produced chemically by the CO and H2 oxidation catalyst heat the CO and H2 oxidation catalyst to speed up its reaching the hydrocarbon light-off temperature of the hydrocarbon oxidation catalyst and hence speed up the hydrocarbon catalyst reaching its light-off temperature.